What Is Keto? A Complete Beginner's Guide
The short answer
A ketogenic diet is a way of eating that keeps carbohydrates so low that your body switches its main fuel source from glucose to fat. In practice that means most of your calories come from fat and protein, while carbohydrates are cut back to roughly 20–50 grams per day. When that happens for a few days, your liver starts converting fat into molecules called ketones, and your body runs on them. That metabolic state is called ketosis, and it is the entire point of the diet.
If you remember only one number from this guide, make it this one: we treat 5 g of net carbohydrates per 100 g of food as the practical keto line for individual foods. It is a screening rule, not a law of physics, but it maps well to how people actually build keto plates. Everything on this site is scored against it, alongside a second, looser line for low-carb eating at 20 g per 100 g.
What you actually eat
A keto plate is built from three groups:
Fat as the main calorie source. Olive oil, butter, avocado, fatty fish, nuts and seeds. Roughly 70–75% of calories, though you will see the range quoted anywhere from 60% to 80% depending on the source. You do not need to drink oil; fat largely comes attached to the foods below.
Protein at moderate levels. Meat, poultry, fish, eggs, cheese. Around 20–25% of calories. Contrary to a common mix-up, keto is not a high-protein diet; protein stays moderate because excess protein can be partially converted to glucose.
Very few carbohydrates. Around 20–50 g per day, and they should come mostly from non-starchy vegetables — leafy greens, broccoli, zucchini, peppers — plus small amounts of berries and nuts. Bread, rice, pasta, potatoes, sugar and most fruit are out or sharply limited.
Everything else — sugary drinks, beer, candy, cakes, most sauces — is simply off the list. This is the part that surprises beginners most: it is not mainly a "meat diet", it is a remove-the-starch-and-sugar diet with fat filling the gap.
Net carbs: the number that matters
Food labels list total carbohydrates, but not all of them digest the same way. Fiber passes through you largely undigested, so people following keto subtract it:
Net carbs = total carbohydrates − fiber
A food with 12 g total carbs and 6 g fiber has about 6 g net carbs. Sugars are not subtracted — they are fully digestible carbohydrate and sit inside the total. One caution: this subtraction rule is a convention, strongest in American labeling where fiber is already included in the total carbohydrate line. On this site we use USDA laboratory data and apply the rule consistently, per 100 g of food, so every verdict you see is on the same scale.
Why per 100 g? Because "one serving" is whatever the manufacturer says it is — a serving of granola can be 30 g or 55 g depending on the brand. A fixed 100 g scale makes foods comparable, and you can scale the number to however much you actually eat. Our net carbs guide walks through the arithmetic with worked examples.
Keto vs low-carb: related but not identical
People use "keto" and "low-carb" interchangeably, but they describe two different levels of restriction. A general low-carb pattern typically allows up to about 100–130 g of carbs per day — roughly 20 g per 100 g of food on our scale. Keto is the strict end: low enough to produce ketones, usually 20–50 g per day, which is why our food line sits at 5 g per 100 g.
The practical consequence: everything that fits keto also fits low-carb, but not the reverse. An apple is fine for a low-carb eater and off the list for keto. Chickpeas, quinoa and most fruit live in that middle zone. This is why every food verdict on FoodCanEat carries two badges — one for keto, one for low-carb — instead of a single yes/no stamp. If you are unsure which pattern fits you, see Keto vs Low-Carb: Which Is Right for You.
What happens in the first two weeks
Cutting carbs below roughly 50 g per day drains your glycogen stores — the glucose stashed in your liver and muscles. Each gram of glycogen is stored with about 3 g of water, so the first week often brings a quick drop in scale weight, most of it water. Alongside it, many people feel genuinely rough for a few days: headaches, fatigue, irritability, trouble concentrating. The community calls it the keto flu, and the main suspects are the loss of sodium and water as insulin levels fall.
The standard countermeasures are unglamorous and effective: salt your food, eat potassium- and magnesium-rich foods (avocado, leafy greens, salmon), and drink water. Our keto flu and electrolytes guide goes through the numbers food by food, because this is one place where the USDA data genuinely helps — electrolyte shortfalls are measurable, not mysterious.
After two to four weeks, most people report the fog lifting and appetite settling. Whether ketosis itself is doing that, or simply the removal of blood-sugar swings, is debated; the experience is common either way.
What the evidence supports — and what it does not
We are a data editorial team, not a medical one, so here is the honest map of what research does and does not show. Keto began about a century ago as a treatment for drug-resistant epilepsy in children, where it remains a legitimate clinical tool supervised by physicians. In the general population, controlled trials lasting 6–12 months have found that low-carb and ketogenic diets can produce meaningful weight loss and improve markers like triglycerides and HDL cholesterol, generally performing at least as well as low-fat alternatives in head-to-head comparisons.
What the evidence does not establish: that keto is superior long-term (few trials run past a year, and differences often shrink), that it is appropriate for everyone (people with certain kidney, liver or pancreatic conditions, pregnant women, and those on glucose-lowering medication need medical supervision), or that ketosis itself — rather than simple calorie reduction — has magic properties. Long-term adherence is the persistent weak point: many people drift back to moderate carb intake, and the best diet in a trial is often the one a person can actually keep.
Treat the claims you see on social media with proportionate skepticism. If a video says keto cures something, look for the trial, not the testimonial. Our What Studies Say column exists precisely to unpack one study at a time, sources cited.
Common beginner mistakes
Hidden carbs in "healthy" foods. Sauces, dressings, nut butters and "low-fat" products often carry more sugar than their labels advertise at first glance. Check per-100-g numbers, not serving-size arithmetic.
Not eating enough fat. People who cut carbs but also fear fat end up in a low-everything state — hungry, tired, and prone to quitting. The fat is supposed to replace the starch.
Ignoring electrolytes. Most early quitting is salt-related. It is the cheapest fix on this list.
Eating "keto products" instead of keto food. A shelf of keto-branded cookies still wants your money. Real food — eggs, fish, meat, vegetables, olive oil — needs no badge.
Expecting the scale to move linearly. Week one is water. Weeks two to six are slower. Judge the diet at the three-month mark, not the three-day mark.
How to use this site
Every day we publish one food verdict: the two badges, the full USDA nutrient profile for that food, how to eat it (or what to eat instead), a recipe, and a note on how the world eats it. Around that we publish scene guides — holidays, office lunches, restaurants — and plain-English research walkthroughs with sources you can check yourself.
Start with the pillar guides, then search any food in our verdict archive. If a food has no verified USDA match, we say so and show no number — an honest gap beats a confident guess.